Laser crystallization of an amorphous silicon film that has been deposited on a substrate, e.g., glass, represents a promising technology for the production of material films having relatively high electron mobilities. Once crystallized, this material can then be used to manufacture thin film transistors (TFT's) and in one particular application, TFT's suitable for use in relatively large liquid crystal displays (LCD's). Other applications for crystallized silicon films may include Organic LED (OLED) and System on a Panel (SOP). In more quantitative terms, high volume production systems may be commercially available in the near future capable of quickly crystallizing a film having a thickness of about 90 nm and a width of about 700 mm or longer. This process may be performed using a pulsed laser that is optically shaped to a line beam, e.g., a laser that is focused in a first axis, e.g., the short axis, and expanded in a second axis, e.g., the long axis. Typically, the first and second axes are mutually orthogonal and both axes are substantially orthogonal to a central ray traveling toward the film. An exemplary line beam for laser crystallization may have a beam width of less than about 20 microns and a beam length of about 700 mm. With this arrangement, the film can be scanned or stepped in a direction parallel to the beam width to sequentially melt and crystallize a film having a substantial length, e.g., 700 mm or more.
In some cases, it may be desirable to ensure that each portion of the silicon film is exposed to a laser energy density that is controlled within a preselected energy density range during melting. In particular, energy density control within a preselected range is typically desired for locations along the shaped line beam, and a somewhat constant energy density is desirable as the line beam is scanned relative to the silicon film. High energy density levels may cause the film to flow resulting in undesirable “thin spots”, a non-flat surface profile and poor grain quality. This uneven distribution of film material is often termed “agglomeration” and can render the crystallized film unsuitable for certain applications. On the other hand, low energy density levels may lead to incomplete melting and result in poor grain quality. By controlling energy density, a film having substantially homogeneous properties may be achieved.
One factor that can affect the energy density within an exposed film is the spatial relationship of the thin film relative to the pulsed laser's depth of focus (DOF). This DOF depends on the focusing lens, but for a typical lens system configured to produce a line beam having a 20 micron beam width, a good approximation of DOF may be about 20 microns.
With the above in mind, it is to be appreciated that a portion of the silicon film that is completely within the laser's DOF will experience a different energy density than a portion of the silicon film that is only partially within the laser's DOF. Thus, surface variations of the silicon film, the glass substrate and the vacuum chuck surface which holds the glass substrate, even variations as small as a few microns, if unaccounted for, can lead to unwanted variations in energy density from one film location to another. Moreover, even under controlled manufacturing conditions, total surface variations (i.e., vacuum chuck+glass substrate+film) can be about 35 microns. It is to be appreciated that these surface variations can be especially problematic for focused thin beam having a DOF of only about 20 microns.
In addition to surface variations, unwanted movements of the film relative to the shaped line beam can also lead to variations in energy density. For example, small movements can occur during stage vibrations. Also, an improper alignment of the stage relative to the shaped line beam and/or an improper alignment of the stage relative to the scan plane can result in an unwanted energy density variation.
Other factors that can lead to a variation in energy density from one film location to another can include changes in laser output characteristics during a scan (e.g., changes in pulse energy, beam pointing, beam divergence, wavelength, bandwidth, pulse duration, etc). Additionally, the location and stability of the shaped line beam and the quality of the beam focus (e.g., shape) during a scan can affect energy density uniformity.
With the above in mind, Applicants disclose several systems and methods for implementing an interaction between a shaped line beam and a film deposited on a substrate.